This invention relates to a d.c. motor unit with a rotational speed detector.
A d.c. motor unit for use in a power-assisted steering mechanism for an automobile, for example, has incorporated therein a rotational speed detector. In a previously proposed d.c. motor unit with a rotational speed detector, the rotational speed detector is mounted on an idle end of the d.c. motor which is the end opposite from the output end from which a rotational output is provided. Thus, the arrangement is such that the d.c. motor has a rotary shaft having an elongated idle-end side portion, on which an inducting element is mounted, an induction coil is mounted to an outer end portion of a free-end side bracket, and an outer end of the induction coil is enclosed by a non-magnetic cover which supports at its inner periphery a magnetic path plate having a plurality of teeth at its inner circumference so that they are placed in an opposing relationship with air gaps therebetween. Also, a permanent magnet is pressure-fitted to an inner surface of the magnetic path plate.
When the induction element is rotated by the d.c. motor, magnetic flux generated by the permanent magnet and passing through the magnetic circuit around the induction coil varies, and an varying output power is induced in the induction coil. Therefore, the rotational speed of the d.c. motor can be measured by detecting the frequency of the output by a suitable measuring means.
With the previously proposed d.c. motor unit with a rotational speed detector, since the induction element is mounted on an elongated portion on the idle end of the rotational shaft and the idle-end side bracket is mounted to an end portion of the yoke of the motor, with the stationary portion of the rotational speed detector mounted to an outer end portion of the bracket, the assembly of the motor is complicated and the overall dimension of the motor is large. Also, the water-resisting structure at the motor idle-end side is disadvantageously complicated.